Current industrial processes for pulping wood and other sources of lignocellulosic material such as annual plants, and processes for bleaching the resultant pulp have evolved slowly over many decades. To remain competitive, the mature pulp and paper industry is seeking new markets for the products produced in pulp mills. It is of particular interest to further refine cellulose and to valorize hemicelluloses and lignin.
Dissolving pulp is a low yield (30-40% by weight on wood) bleached chemical wood pulp that has high alfa cellulose content (>90%). This pulp has special properties, such as a high level of brightness and high purity. Dissolving pulp is used for production of regenerated cellulose products. The dominant process for manufacturing regenerated cellulose fibers, the viscose process, is suffering from a high environmental burden and high energy demand. The viscose process is using large quantities of carbon disulfide a chemical often contaminated with foul-smelling impurities, such as carbonyl sulfide, hydrogen sulfide and organic sulfides. Even the best of current technology is unable to suppress the odors emitted in viscose plants. Furthermore there is no efficient chemicals recovery process for recovery of spent dissolving and coagulation chemicals.
The NMMO (N-Methylmorpholine oxide) process, a rather new non-derivatizing process for producing regenerated cellulose fibers is emerging as an alternative to the viscose process, however, recovery of the NMMO solvent is complicated, energy demanding and costly. The Chinese patent application CN 101280476 is directed to a new method for recycling of NMMO solvent using cationic and anionic resins.
The European patent application EP 1900860 is directed to a process for dissolving cellulose in a sodium hydroxide urea mixture. While this process may have an advantage in comparison to the viscose and NMMO processes there is no suggestion on how to recover the dissolving/coagulation chemicals.
Dissolving types of pulps, whether they are produced by a prehydrolysis kraft process or sulfite process, are traditionally used for manufacturing of viscose or cellulose derivatives such as cellulose esters, rayon fibers and cellophane. Rayon is a soft textile material, used in mostly tops, coats and jackets. Viscose material can be produced either from dissolving grade pulp or from cotton linter fibers. The manufacturing process starts by treating the fibers with sodium hydroxide (mercerization). The mercerized pulp is thereafter mixed with carbon disulfide to form cellulose xanthate, a cellulose ester. The cellulose xanthogenate is dissolved in sodium hydroxide forming a viscous cellulose solution. The cellulose solution or viscose is extruded into an acidic bath either through a slit to make cellophane, or through a spinneret to make rayon. In the acidic environment the xanthogenate ester is decomposed into cellulose and sulphurous compounds. A portion of the carbon disulfide is recovered and recycled to treat new cellulose.
European Patent EP1521873 is directed toward a process for the manufacture of solid regenerated viscose fiber describing certain new features of the traditional viscose process.
The viscose process was developed well over hundred years ago and the process still has a dominant position on the market for production of regenerated cellulose. For more details on the viscose process (and NMMO process) reference is made to “Regenerated Cellulose Fibres” The Textile Institute, Ed. Calvin Woodings, Cambridge 2001. (ISBN 1 85573459 1)
Dissolving pulp can be manufactured by alkaline (kraft, soda) and acidic (sulfite, bisulfite) pulping processes. In the kraft process the cooking liquor is made up of sodium hydroxide and sodium sulfide, in a nonsulfur soda pulp mill the sodium sulfide is, in some locations, at least partly, replaced with anthraquinone. Sulfite mills uses sodium sulfite or magnesium sulfite/bisulfite as the active cooking chemicals. The chemicals recovery cycle in a pulp mill include a recovery boiler, evaporation plant, sulfur dioxide recovery units (for sulfite mills) and recausticising plant (alkaline pulp mills). For a detailed description of kraft, soda and sulfite chemical pulping reference is made to “Chemical Pulping” Book 6A, Ed. Johan Gullichsen, 2000. (ISBN 952-5216-06-3) and “Pulp and Paper Manufacture” Volume 4. Sulfite Science & Technology” Ed. by O. V. Ingruber, 1985. (ISBN 1-919893-22-8).
When the objective of the cellulose pulping operations is to produce a dissolving type of pulp the target physical quality parameters for the product are different than target quality parameters for paper pulp. Tear and tensile strength is no longer important while cellulose pulp purity is of essence (low lignin content, low metals and ash content.
It is apparent that there is a need for a new and more efficient cellulose dissolving process to replace the traditional viscose process.